1. Field of the Invention
The present invention relates to a color picture tube and a method for manufacturing the same. More specifically, the present invention relates to a shape of a shadow mask provided in a color picture tube and a method for forming the shadow mask.
2. Description of Related Art
The following is a description of a conventional typical color picture tube. FIG. 5 is a schematic sectional view showing an exemplary structure of the conventional color picture tube. The color picture tube includes an envelope having a panel 1 and a funnel 2, and a space inside the envelope is maintained under vacuum. The panel 1 has a face portion 1A and a skirt portion 1B formed on the periphery of the face portion 1A. An outer surface of the face portion 1A is substantially rectangular. The funnel 2 is joined with the skirt portion 1B of the panel 1 on its larger dimension side.
An inner surface of the face portion 1A is provided with a phosphor screen 3. The phosphor screen 3 has a plurality of black light absorption layers and a plurality of tricolor phosphor layers. Each of the black light absorption layers has a striped shape extending along a minor axis direction of the panel 1, and the plurality of black light absorption layers are arranged in parallel with a predetermined gap therebetween in a major axis direction. Each of the plurality of tricolor phosphor layers has a striped shape extending along the minor axis direction of the panel 1 and is arranged sequentially in the gap between the plurality of black light absorption layers.
An electron gun 4 is provided in a space inside a cylindrical neck portion 2A of the funnel 2. The electron gun 4 usually is an in-line electron gun, which emits in a tube axis direction three electron beams consisting of a center beam and a pair of side beams on both sides aligned in a horizontal axis direction (the major axis direction).
Further, a deflection yoke (a deflector) 5 is mounted on an outer peripheral surface of the funnel 2. The deflection yoke 5 has a horizontal deflection coil and a vertical deflection coil for forming a deflection magnetic field and deflects the three electron beams emitted from the electron gun 4 by a magnetic action.
In the space inside the envelope, a shadow mask 106 is provided so as to be spaced away from and face the phosphor screen 3. The shadow mask 106 is broadly grouped into a dome-like press-formed mask formed by press work and a substantially flat tension mask stretched and fixed by a mask frame 7. It is noted that, when simply referring to a shadow mask in the present specification, it means a press-formed mask.
The mask frame 7 for fixing the shadow mask 106 is attached to the panel 1 in a freely detachable manner by engagement through an elastic support (not shown) fixed to the mask frame 7 and a stud pin (not shown) provided in the skirt portion 1B of the panel 1 so as to protrude therefrom.
Now, an exemplary structure of the shadow mask will be described in detail. FIG. 6 is a schematic perspective view showing an exemplary structure of the shadow mask. As shown in FIG. 6, the shadow mask 106 has a perforated portion (a useful portion) 61 in which many apertures through which the three electron beams for displaying an image pass (electron beam passing apertures, not shown) are formed in a predetermined arrangement, a non-perforated portion (a nonuseful portion) 162 formed on the periphery of the perforated portion 61, and a skirt portion 63 formed on the periphery of the non-perforated portion 162. The skirt portion 63 is fixed to the mask frame 7 (see FIG. 5) by welding or the like. As shown in the figure, an axis that is parallel with a longer side of the substantially rectangular perforated portion 61 and perpendicular to a tube axis is set as a major axis, whereas an axis that is perpendicular to the major axis and the tube axis is set as a minor axis. Usually, the skirt portion 63 is welded to the mask frame 7 at positions where the minor axis crosses the skirt portion 63 or a vicinity thereof, positions where the major axis crosses it or a vicinity thereof, and diagonal edges or a vicinity thereof.
The many apertures in the perforated portion 61 individually form a plurality of aperture trains that are substantially parallel with the minor axis direction. The plurality of aperture trains are arranged along the major axis direction at a predetermined pitch. The pitch of the aperture trains varies from the center of the perforated portion 61 to the periphery thereof in the major axis direction and usually increases gradually from the center toward the periphery along the major axis direction. A plurality of apertures forming each aperture train are aligned substantially in parallel with the minor axis direction via bridges (not shown). Positions of apertures in adjacent aperture trains in the minor axis direction are shifted by ½ of the pitch of the apertures in the minor axis direction. In other words, the apertures in the plurality of aperture trains are arranged in a so-called staggered manner. Each of the apertures has an elongated substantially rectangular shape, with its longitudinal direction being parallel with the minor axis direction and its lateral direction (width direction) being parallel with the major axis direction. Each of the apertures is a communication aperture in which a larger aperture (not shown) that opens to the surface on the side of the phosphor screen 3 (see FIG. 5) and a smaller aperture (not shown) that opens to the surface on the side of the electron gun 4 (see FIG. 5) are in communication with each other.
Each of the three electron beams passes each aperture of the perforated portion 61 and then reaches only a phosphor layer with a specific color among the tricolor phosphor layers constituting the phosphor screen 3. In other words, the shadow mask 106 is provided for color selection of the three electron beams.
In the color picture tube shown in FIG. 5, the three electron beams emitted from the electron gun 4 are deflected in the major axis direction and the minor axis direction by the deflection magnetic field generated by the deflection yoke 5, pass through the apertures of the shadow mask 106, and scan the phosphor screen 3 in the major axis direction and the minor axis direction. In this manner, a color image is displayed on the phosphor screen 3.
For improving the visibility of displayed images, the screen has become increasingly flatter, so that the outer surface of the face portion 1A has become substantially flat (has a radius of curvature of at least 10000 mm) and the inner surface of the face portion 1A also has achieved a high flatness. Since the curved shape of the shadow mask 106 (a curved mask surface) generally depends on the shape of the inner surface of the face portion 1A, the flattening of the inner surface of the face portion 1A also increases the flatness of the curved shape of the shadow mask 106.
Herein, a general method for producing the shadow mask 106 will be explained. A rolled-up elongated mask base is unrolled and then provided with many apertures for color selection by a photo-etching treatment while being carried in its longitudinal direction. Subsequently, the mask base is cut into a predetermined size, thus producing a flat mask. After the flat mask is annealed, it is subjected to press work, thereby deforming the flat mask plastically. By the processes described above, the shadow mask 106 is produced.
The following is a detailed description of the press work for producing the shadow mask 106. FIGS. 7A to 7F are drawings for describing the press work. First, as shown in FIG. 7A, an annealed flat mask 16 is put into a press working machine. Incidentally, the annealing is performed for simplifying the press work. The press working machine includes a mask die that is roughly divided into a punch 11, a knockout 12, a die 13 and a blank holder 14. A lower surface P1 of the punch 11 has a shape corresponding to the shapes of the perforated portion and the non-perforated portion of the shadow mask to be produced. The knockout 12 facing the punch 11 has a protruding portion 12a for clamping the flat mask 16 between the protruding portion 12a and the punch 11 and a recessed portion 12b for receiving the perforated portion that extends toward the recessed portion 12b at the time of pressing. An upper surface P2 of the protruding portion 12a has a shape corresponding to the shape of the non-perforated portion of the shadow mask to be produced. The die 13 and the blank holder 14 that face each other are arranged around the punch 11 and the knockout 12.
Next, as shown in FIG. 7B, the blank holder 14 is lowered, so that the die 13 and the blank holder 14 hold the flat mask 16. Subsequently, as shown in FIG. 7C, the punch 11 is lowered, thus deforming the perforated portion and the non-perforated portion so as to extend downward. Thereafter, as shown in FIG. 7D, the blank holder 14 alone is lifted. Then, as shown in FIG. 7E, the punch 11 and the knockout 12 are lowered while keeping the flat mask 16 clamped therebetween, thus forming the skirt portion with the punch 11 and the die 13. Finally, as shown in FIG. 7F, the punch 11 is lifted, and the completed shadow mask 106 is taken out from the press working machine.
In press work, as the curved mask surface of the shadow mask 106 to be produced becomes flatter (the radius of curvature increases), it is more difficult to deform the flat mask 16 plastically. This is because, as the curved mask surface becomes flatter, the difference between the length along the surface of the perforated and non-perforated portions of the flat mask 16 before press work and the arc length along the surface of the perforated and non-perforated portions of the shadow mask 106 after press work decreases, thereby reducing the amount that the perforated portion is stretched during the press work. Accordingly, in the case of producing a highly-flat shadow mask 106, a technology has been adopted in which one or more lines of beads (not shown) are formed in the non-perforated portion, making it easier to achieve plastic deformation (see JP 7(1995)-29505 A, JP 8(1996)-106856 A and JP 2002-313254 A, for example).
Here, the shadow mask having the beads will be described in detail. FIG. 8 is a schematic front view showing an exemplary structure of a conventional shadow mask having two lines of beads. FIG. 9 is a schematic sectional view showing in detail a part of the exemplary structure of the conventional shadow mask having the two lines of beads. Incidentally, FIG. 9 shows a cross-section taken along the minor axis of the shadow mask. As shown in FIGS. 8 and 9, an outer bead 108 is formed in the non-perforated portion 162 so as to surround the perforated portion 61, and an inner bead 109A and an inner bead 109B respectively are formed in a pair of longer side portions 162A and a pair of shorter side portions 162B of the non-perforated portion 162 so as to be located between the perforated portion 61 and the outer bead 108.
For press work for producing the shadow mask shown in FIGS. 8 and 9, the press working machine shown in FIG. 7A uses a mask die in which the upper surface P2 of the protruding portion 12a of the knockout 12 is provided with two lines of ridge-like projections (not shown) and the lower surface P1 of the punch 11 is provided with two lines of groove-like depressions (not shown) having a shape corresponding to the projections of the knockout. By forming the beads with the two lines of projections of the knockout 12 and the two lines of depressions of the punch 11, the perforated portion 61 is stretched strongly outward (in a direction away from the tube axis) so as to increase the extending amount at the time of pressing, making it easier to achieve plastic deformation.
In press work, generally, the flat mask 16 serving as an object to be processed, in particular, the non-perforated portion 162 thereof, has to be fixed firmly. Accordingly, a portion of the mask die corresponding to the non-perforated portion 162 partially has been processed for engagement (rubbing for engagement). In order to fix the flat mask 16 as a whole uniformly, the rubbing for engagement is provided in a region in the mask die completely surrounding the portion corresponding to the perforated portion 61. The region subjected to the rubbing for engagement in the mask die is referred to as a “rubbed region for engagement.” Conventionally, this rubbing for engagement has been provided in the region of the mask die corresponding to a region between the perforated portion 61 and the bead on the inner most side (the side of the perforated portion). It should be noted that a region in the mask die other than the rubbed region for engagement is left as a machine-processed surface.
The rubbing for engagement is a fine hand finishing on a machine-processed surface by an individual worker for allowing the punch 11 (see FIG. 7A) and the protruding portion 12a of the knockout 12 (see FIG. 7A) to contact closely. The rubbing for engagement is carried out as follows. Pounce is applied to the rubbed region for engagement of one of the punch 11 and the knockout 12. In this state, stamping is performed without inserting the flat mask 16 (see FIG. 7A). The adhesion of the pounce to the other rubbed region is observed, and when there is a portion to which no pounce adheres or an unevenness of the adhering pounce, the corresponding portion of the punch 11 or the knockout 12 is rubbed manually. This is repeated until the adhesion of the pounce becomes uniform.
The rubbing for engagement is performed locally in the rubbed region for engagement by a manual work. Thus, although the rubbing for engagement allows the upper and lower mask dies to contact closely within the rubbed region for engagement, it may deteriorate a working accuracy of the rubbed region for engagement of the mask die with respect to the surface shape as a whole. In other words, the rubbing for engagement causes a work unevenness in the mask die. For example, there are some cases where a surface that is supposed to be flat is processed into a curved surface or a wavy uneven surface by the rubbing for engagement. Even in such cases, as long as the rubbed region for engagement of the punch 11 and that of the knockout 12 have surface shapes corresponding to each other, it still is possible to allow them to contact closely. However, in this case, because of the work unevenness in the rubbed region for engagement, wavy wrinkles are generated in the shadow mask to be formed using this mask die.
Conventionally, the rubbed region for engagement has been located in the region of the mask die corresponding to the region between the perforated portion 61 and the inner most bead, and at the time of forming the curved surface of the perforated portion 61, the flat mask 16 has been clamped at this rubbed region for engagement. Thus, owing to the above-described work unevenness in the rubbed region for engagement in the mask die, the wrinkles have reached the perforated portion 61. In particular, a wide-type color picture tube whose aspect ratio (major axis length:minor axis length) is 16:9 is more markedly affected by the work unevenness because of its higher flatness of the perforated portion 61, compared with a color picture tube whose aspect ratio is 4:3.
The wrinkles in the perforated portion 61 are more likely to be generated in a central part of the perforated portion 61 than in the peripheral part thereof. This is because the radius of curvature of the perforated portion 61 in the minor axis direction is largest in the vicinity of the minor axis and decreases with distance away therefrom along the major axis and the radius of curvature thereof in the major axis direction is largest in the vicinity of the major axis and decreases with distance away therefrom along the minor axis. Furthermore, the generation of wrinkles in the central part of the perforated portion 61 is affected more greatly by the work unevenness in the portion of the mask die corresponding to the longer side portion 162A of the non-perforated portion 162 of the shadow mask rather than by the work unevenness in the portion of the mask die corresponding to the shorter side portion 162B thereof. This is because the radius of curvature of the central part of the perforated portion 61 generally is larger in the minor axis direction than in the major axis direction.
Even when very shallow wrinkles are generated in the perforated portion 61, the durability of the curved shape of the shadow mask against impact or the like (mask strength) decreases, so that the shadow mask itself becomes easy to deform by an external force. The deformation of the shadow mask has reduced yield in a manufacturing process of the color picture tube and deteriorated the image quality in the color picture tube. Moreover, when wrinkles are generated in the perforated portion 61, it is more likely that vibrations due to an external impact, a sound or the like causes vibrations (mask vibrations) in the perforated portion 61. Because of the generated mask vibrations, swaying images are formed when the images are displayed, thus lowering the image quality. Especially in the case of a color picture tube for television, since the shadow mask (the screen size) is large, the lowering of mask strength and the generation of mask vibrations caused by the wrinkles in the perforated portion 61 become more conspicuous.